20 THE PHOTOGRAPHIC IMAGE, BROCK 
4. Resolution versus contrast transmission and other test methods. 
4.1. General. 
At the present time the resolution test is probably used more than any other for 
assessing photogrammetric lenses, cameras, and emulsions. In view of its undoubted dis- 
advantages and limitations, is there any prospect of it being replaced by some new test, 
such as the contrast transmission just discussed, or the Eastman “acutance”, the “effec- 
tive spot-size" of Hopkins, or something else? Or will all these testing methods continue 
side by side, since each tells us something different, or tells it more easily? Will it not 
be possible to change the present situation, in which full knowledge of a camera’s per- 
formance requires many tests of different kinds and reliance on only one involves as- 
sumption and a certain amount of risk? 
The writer believes that the resolution test is likely to persist, and that frequency 
response techniques will not be in common use for several years. Since this is a contro- 
versial matter, some space is devoted to an appreciation of the status of the resolution 
test, with its problems of standardisation and its inherent limitations. 
Without personal experience of the Hopkins test or acutance methods it is hard to 
judge them properly, but the following opinions are offered. 
The proposal to define the lens performance by the diameter of the circle containing 
90% (or some other fraction) of the total light energy has some attraction, especially if 
it could be linked to an effective spot size for the emulsion. But the results will not be 
very easy to interpret, and the instrumental measuring difficulties appear to be nearly 
as great as in frequency response. It seems better, therefore, to concentrate effort on the 
latter. 
The acutance test gives no result that can be fitted into a general concept of camera 
performance, in the way that resolution or frequency response tests can be used to pre- 
dict the kind of detail likely to be visible from a certain height. The conditions of deter- 
mination (opaque knife-edge) are so remote from the imaging conditions of air photo- 
graphy that the results can give little guidance on the practical behaviour of the camera 
or emulsion. The difficulties of determination again seem to be comparable with those 
of contrast transmission, and the information is much less valuable. Therefore, in spite 
of the fact that edge-sharpness is obviously of fundamental importance in the study of 
images, it does not seem a promising approach in this case. 
4.2. The resolution test. 
4.2.1. Is it a true index of performance? 
Apart from its simplicity, the great appeal of the resolution test is undoubtedly that 
it follows through the actual operations of taking photographs, and gives about the best 
conceivable summing-up, in simple objective form, of the complex subjective impressions 
gained when looking at them under magnification. The fact of having a permanent self- 
explanatory record to point at has been of immense service to the user, especially in the 
early days, when resolution tests were his only protest and protection against the older 
school of lens designers. There, beyond argument, was a clear demonstration of the film 
being limited by the lens, whatever might be seen in the aerial image. It is perhaps wise 
to reflect on this when arguing that resolution is not what we want to know — we may 
not be right on the target, but we are much closer than we used to be. When a test has 
been in use for some twenty-five years, and has stimulated the efforts that gave vastly 
improved lenses, it is not without merit. 
It can be admitted that the resolution end-point is only one characteristic of a photo- 
graphie image, and no one would dispute that fuller information is desirable. The ques- 
tion is whether the resolution test is positively misleading, as suggested by the Eastman 
work on acutance. This is largely a matter of what we wish the photograph to do. If it 
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